1. Field of the Invention
The present invention relates to a lithium battery including a solid electrolyte layer.
2. Description of the Related Art
Lithium-ion secondary batteries (hereinafter, simply referred to as “lithium batteries”) have been used as a power supply of relatively small electrical devices such as portable devices. Lithium batteries include a positive electrode layer, a negative electrode layer, and an electrolyte layer that mediates conduction of lithium ions between the positive electrode layer and the negative electrode layer.
Recently, as such lithium batteries, all-solid-state lithium batteries in which an organic electrolyte solution is not used for conducting lithium ions between a positive electrode and a negative electrode have been proposed. In all-solid-state lithium batteries, a solid electrolyte layer is used as an electrolyte layer. Accordingly, all-solid-state lithium batteries can eliminate disadvantages caused by the use of an organic solvent-based electrolyte solution, for example, a safety problem caused by leakage of an electrolyte solution and a heat-resistance problem caused by volatilization of an organic electrolyte solution at high temperatures higher than the boiling point of the electrolyte solution. As the solid electrolyte layer, a sulfide-based substance having a high lithium-ion conductivity and excellent insulating property is widely used.
While such all-solid-state lithium batteries including a solid electrolyte layer have the above-described advantages, the air-solid-state lithium batteries have a problem of a low capacity (i.e., unsatisfactory output characteristic) as compared with lithium batteries including an organic electrolyte solution. The cause of this problem is that since lithium ions are more easily drawn to oxide ions of a positive electrode layer than sulfide ions of the solid electrolyte layer, a layer where lithium ions are lacking (depletion layer) is formed in an area at the positive electrode layer side of the sulfide solid electrolyte (refer to Advanced Materials 2006. 18, 2226-2229 (Reference 1)). This depletion layer has a high electrical resistance because of the lack of lithium ions, and thus decreases the capacity of the battery.
To solve the above problem, according to a technique disclosed in Ref. 1, a surface of a positive electrode active material is coated with a lithium-ion conductive oxide. This coating limits the migration of lithium ions and suppresses the formation of the depletion layer in a sulfide solid electrolyte layer. As a result, an improvement in the output characteristic of a lithium battery is realized.
However, the lithium battery disclosed in Ref. 1 is disadvantageous to the demand expansion of lithium batteries due to the recent development of portable devices because the productivity for the lithium battery is low. Specifically, according to Ref. 1, coating is performed on a surface of an active material by electrostatic atomization. This coating performed by electrostatic atomization is technically difficult and complex. That is, the production cost of the lithium battery disclosed in Ref. 1 is high, and the production efficiency thereof is also low. Accordingly, it is difficult to meet the requirement of demand expansion of lithium batteries.
Furthermore, in recent years, there has been a demand for further reducing the thickness of lithium batteries used in portable devices. However, the lithium battery disclosed in Ref. 1 is disadvantageous in that it is difficult to reduce the thickness of the battery while maintaining the capacity. Specifically, in the lithium battery disclosed in Ref. 1, the amount of positive electrode active material occupying a positive electrode layer is decreased by an amount corresponding to the coating formed on the surface of the positive electrode active material. In addition, in the lithium battery disclosed in Ref. 1, the positive electrode layer is composed of a powdery positive electrode active material the surface of which has been coated, and it is believed that a binding agent for binding the powdery active material is contained in the positive electrode layer. As a result, the amount of active material occupying the positive electrode layer is decreased by an amount corresponding to the binding agent. That is, in order to maintain the capacity of the lithium battery disclosed in Ref. 1, the thickness of the positive electrode layer must be increased.